I'm thinking of starting a project for a low budget (add emphasis) RC modules to control some very simple drones, similar to cars. I've done a little research on frequencies, and tend to favor 49Mhz bands.

There will need to be 4 separate units, operating at the same time. They will be extremely local, so they would be very low power. That will take care of legal requirements nicely.

I'm not too worried about the digital encoding/decoding, that is actually very easy. Making a stable transmitter and a receiver for bottom dollar is a bit more intimidating, but not especially so.

Before I invest some serious skull sweat and solder I was wondering if folk knew what else was out there.

Had not completely decided, but my thought was simple on/off encoding for the radio pulses. No servos, just brute simple. I'll draw up a rough schematic for the RF section of the transmitter and show it in a bit. I figure you could do simple tank style driving in around 4 bits, non proportional controls. Toggle switches on a box.

We have had a new combat robotics group started locally. I was thinking of making 3-4 really cheap pushy bots for the kids who come to the meeting to play with. No weapons on this batch, all identical except for color (where the 3D printer comes in).

I'm not sure exactly why, but most of my kids inexpensive RC toys use IR, and they actually work really well. I'm not sure if this is because it's cheaper/easier to make, or if it just saves a ton on FCC approval stuff. Anyway, I thought it was interesting and they must be doing it for a reason.

Also unless you're just doing it for fun, commercial RF receivers are dirt cheap these days, for example:

I know you said you've decided on 49MHz but I thought I'd chime in as I'm trying to learn about Multiplexing so I'm experimenting with some TX/RX units I got off ebay (making the TX + RX seemed way over my head ). The ones I got were 433MHz (one of our ISM bands but frequency can vary) with variable transmitter power depending on voltage (20 mW@5V apparently and TTL logic). Listings state that the bandwidth is 2MHz and its modulation is ASK/OOK but I had to check through a few of the listing to get the complete picture of the specs. I just took a punt on these ones as I'm just messing about with them to get my footing, I'm not sure how useful they would be to your application but thought I'd link them anyway. Hope it goes well

If you are going to operate bots at a gathering or a match I suggest that you consider IR. At gatherings concerning radio control they are pretty adamant about you not operating a transmitter when something else is going on. At the local RC modelers air strip you have to turn in your transmitter when you arrive on site. You get it back when it is your time to fly.

In your schematic, the modulation transistor will draw max current through the inductor when on. Maybe move it to the base side.

2. The stack multiple channels by switching to higher modulation frequencies (usually AF frequencies) and use AF bandpass filters. So low frequency pulse duty cycle controls drive fwd/rev, high frequency pulse duty cycle controls steering. Or with a treaded vehicle, one AF frequency controls fwd/rev/speed of one side and the other AF frequency controls the other side.

3. The cheapo receivers are so unselective they usually support only two units, on different bands -- 27MHz vs. 49MHz.

So how selective do you plan to make your receivers, and how stable and tuned are you going to make that transmitter?

I think you'll have a lot more luck with 433MHz digital systems or like others have said, IR.